THE INVOLVEMENT OF CYP1A2 AND CYP3A4 IN THE METABOLISM OF CLOZAPINE

Aims Clozapine (CLZ), an atypical neuroleptic with a high risk of caus ing agranulocytosis, is metabolized in the liver to desmethylclozapine (DCLZ) and clozapine N-oxide (CLZ-NO). This study investigated the in volvement of different CYP isoforms in the formation of these two meta bolites. Methods Human liver microsomal incubations, chemical inhibito rs, specific antibodies, and different cytochrome P450 expression syst ems were used. Results K-m and V-max values determined in human liver microsomes were lower for the demethylation (61+/-21 mu M, 159+/-42 pm ol min(-1) mg protein(-1) mean+/-s.d.; n=4), than for the N-oxidation of CL (308+/-1.5 mu M, 456+/-167 pmol min(-1) mg protein(-1); n = 3). Formation of DCLZ was inhibited by fluvoxamine (53+/-28% at 10 mu M), triacetyloleandomycin (33+/-15% at 10 mu M), and ketoconazole (51+/-28 % at 2 mu M) and by antibodies against CYP1A2 and CYP3A4. CLZ-NO forma tion was inhibited by triacetyloleandomycin (34+/-16% at 10 mu M) and ketoconazole (51+/-13% at 2 mu M), and by antibodies against CYP3A4. T here was a significant correlation between CYP3A content and DCLZ form ation in microsomes from 15 human livers (r=0.67; P=0.04). A high but not significant correlation coefficient was found for CYP3A content an d CLZ-NO formation (r=0.59; P=0.09). Using expression systems it was s hown that CYP1A2 and CYP3A4 formed DCLZ and CLZ-NO. K-m and V-max valu es were lower in the CYP1A2 expression system compared to CYP3A4 for b oth metabolic reactions. Conclusions It is concluded that CYP1A2 and C YP3A4 are involved in the demethylation of CLZ and CYP3A4 in the N-oxi dation of CLZ. Close monitoring of CLZ plasma levels is recommended in patients treated at the same time with other drugs affecting these tw o enzymes.